1. Field of the Invention
The present invention relates to a fabricating method of a composite material for compounding two materials differing in melting point and not mixing with each other in solid solution, such as W (tungsten) and Cu (copper), and a heat conductive material and fabricating method of heat conductive material.
2. Description of the Related Art
In the equipment possessing ultrahot molten metal field as represented by active metal melting crucible and heat receiving plate, it is often indispensable to use a material withstanding electron beam or plasma having high temperature and high energy density, that is, the beam target material.
Since the beam target material is used in rugged condition, it is required to satisfy the following two characteristics: (1) sufficient resistance to heat directly beneath heat source rising in temperature, (2) excellent heat conductivity and cooling characteristic. The second point is needed because the opposite side of the heat source is generally cooled by some means.
In a single material, however, the heat resistance and heat conductivity cannot be considered separately, and when either one is determined, the other is automatically determined, of course, within a limited range. Accordingly, hitherto, in order to enhance the both characteristics together, it is attempted to composite materials. One of such attempts is to fabricate a composite material excellent in both heat resistance and heat conductivity, by combining W which is high in melting point among metals and Cu which is excellent in heat conductivity.
When compounding W and Cu, however, since they are not mixed with each other in solid solution, the bonding method is limited, and simple gluing or brazing or other mechanical bonding methods are mainly employed.
If such compound of W and Cu is used in high temperature field, the difference in thermal expansion between the two is very large. Specifically, W has thermal expansion factor of 4.5.times.10.sup.6 /K, while Cu has that of 17.1.times.10.sup.6 /K, and the generated thermal stress is extremely large. Accordingly, when W and Cu are merely brazed and compounded, cracks are likely to be formed in W which has the smaller thermal expansion factor, due to tensile stress when heating, as well as peeling due to thermal stress caused in the interface of W and Cu. Such cracking and peeling will lower the total thermal conductivity, which may lead to temperature rise of materials, or meltdown accident in worst cases.
It is therefore attempted to obtain gradient materials with structure control (hereafter referred to as "gradient material" recently by mixing two powders, laminating by varying their mixing ration, and sintering the laminates.
This technique is capable of obtaining gradient materials if the melting points of two powders to be mixed are similar to each other, but when extremely different, only one material is molten while the other is not, and it is difficult to fabricate a gradient material with structure control.